This invention relates generally to automotive exhaust systems and, in particular, to diagnostic techniques for determining the NOx storage capacity of a NOx adsorber used in such exhaust systems. This invention also relates to techniques for operating an internal combustion engine in accordance with the storage capacity of a NOx adsorber connected in the exhaust system so as to maximize performance of the engine while minimizing the NOx exhausted into the environment.
Exhaust gas treatment devices have been used as original equipment by automotive manufacturers for many years as a way to reduce the HC, CO, and NOx emissions from automotive internal combustion engines. Initially, thermal afterburning was used to reduce emissions by burning the unburnt fuel contained in the exhaust gas. However, this approach has been found to only provide limited benefits and is not useful in reducing NOx emissions to acceptable levels. This technique has primarily been replaced by catalytic converters which utilize a monolithic structure containing noble metals (Pt, Rh, Pd) to provide catalytic afterburning of the engine emissions. Today""s more advanced systems utilize a three-way catalytic converter that is capable of simultaneously reducing the emissions of HC, CO, and NOx. To maximize the efficiency of these three-way converters, the engines are typically run at stoichiometry; that is, they are run at an air/fuel ratio in which the amount of air (oxygen) inducted into the cylinder is no more and no less than required to burn all of the injected fuel. One problem with this mode of engine operation is that it is not always possible or desirable to operate the engine at stoichiometry. Rather, for purposes of maximizing fuel economy, it is often desirable to operate the engine in a lean combustion condition in which the amount of intake air is greater than is needed to burn the injected fuel. Conversely, during engine warm up and during periods of acceleration when torque is required, it is desirable for driveability to operate the engine in a rich combustion condition in which the amount of fuel injected is greater than the amount of fuel that the inducted air can burn.
More recently, NOx adsorbers have been developed which store NOx during periods of lean engine combustion (i.e., excess air) and then periodically release the NOx during periods of rich combustion (i.e., excess fuel) so that the NOx can be catalytically reduced due to the presence of excess HC, CO, and H2. See, for example, U.S. Pat. No. 5,473,887 to Takeshima et al. which discloses an exhaust purification system that operates to reduce NOx emissions by periodically running the engine at a rich combustion condition to release and catalytically reduce the NOx stored during the periods of normal lean engine operation. Since the initial development of NOx adsorbers, many refinements have been developed to help further regulate and reduce the NOx emissions from the exhaust system. For example, U.S. Pat. No. 5,483,795 to Katoh et al. discloses a system in which an exhaust gas oxygen sensor (referred to herein as an O2 sensor) is placed downstream of the NOx adsorber to determine the length of time needed to release all of the stored NOx during rich engine operation. The Katoh et al. system works on the principle that after the engine is switched from lean to rich combustion, there is a delay before the downstream O2 sensor detects a rich combustion condition, with this delay being due to the release of NOx which reacts with the HC, CO, H2 contained in the exhaust. Thus, switching of the downstream O2 sensor to a voltage indicative of a rich condition is used as a signal that all of the NOx stored in the NOx adsorber has been released and that the engine can therefore be returned to lean operation.
The amount of NOx that can be stored in a NOx adsorber during any one lean cycle is dependent upon the state, volume, and temperature of the NOx adsorber. Over time, NOx adsorbers can deteriorate due to, for example, poisoning from sulfur oxides. Accordingly, exhaust purification systems have been suggested which determine the degree of deterioration and respond accordingly. For example, U.S. Pat. No. 5,577,382 to Kihara et al. discloses a system in which the peak magnitude of a downstream O2 sensor is used to determine when the NOx adsorber has sufficiently deteriorated that it needs to be regenerated by running the engine rich until the stored sulfur oxides are released. Similarly, U.S. Pat. No. 5,735,119 to Asanuma et al. discloses a system which detects the degree of deterioration of the NOx adsorber, again using an O2 sensor. As the degree of deterioration increases, the system decreases the length of engine operating time at both the lean and rich combustion conditions. This has the effect of reducing the amount of NOx supplied to the NOx adsorber (i.e., the amount of engine-out NOR) before the rich regeneration of the NOx adsorber, and also has the effect of reducing the length of the rich regeneration, since there will be less NOx adsorbed and hence, less regeneration time required.
Another approach for achieving efficient use of a NOx adsorber is to determine the amount of engine-out NOx as the engine is operated in its lean combustion mode and to then switch to rich combustion when the total engine-out NOx supplied during the current lean period equals the adsorption capacity of the NOx adsorber. See, for example, U.S. Pat. No. 5,437,153 to Takeshima et al. Engine-out NOx can also be used along with other variables to determine the state of the NOx adsorber. See, for example, U.S. Pat. No. 5,743,084 to Hepburn which discloses a method for monitoring a NOx trap in which upstream and downstream O2 sensors are used to determine the amount of NOx stored in the NOx trap, with the stored NOx amount being used along with an estimated engine-out NOx to determine the storage efficiency of the NOx trap. Thereafter, the period of lean engine operation is reduced as the determined storage efficiency drops. This system is based upon the same essential principle as that disclosed in the above-noted Katoh et al. patent; namely, that the delay in switching of the downstream O2 sensor when engine operation changes from lean to rich is due to the release of NOx stored in the NOx trap, and for a given temperature, this delay time provides a quantitative measure of the amount of NOx released (and therefore previously stored) in the NOx trap.
One problem with using the O2 sensor delay time as a measure of the amount of stored NOx is that the delay time is not only due to the release of stored NOx, but also to the release of oxygen stored during the lean period. Thus, in U.S. Pat. No. 5,713,199 to Takeshima et al., the amount of delay time due to the release of oxygen is determined and is subtracted from the total O2 sensor delay to determine the amount of delay due to the release of NOx only. This truer delay time can then be used to more accurately estimate the amount of NOx stored during the previous lean period. The amount of delay time due to the release of oxygen is determined by a separate lean/rich cycle in which the engine must be operated lean for a period of time that is long enough to fully store the oxygen in the NOx adsorber, but is short enough that no appreciable NOx has yet been stored. The engine is then switched to rich operation, and the delay time between the upstream and downstream O2 sensors is taken as a measure of the delay due to the release of oxygen only in the NOx adsorber. This delay is later subtracted from the total delay when the engine is operated in its normal lean/rich cycle. While providing a more accurate measurement of the delay due to the release of NOx only (and, thus a more accurate measurement of the amount of stored NOx), this system requires a separate abnormal lean/rich engine cycle to determine the oxygen release delay time, and this cycle may need to be periodically repeated as the state of the NOx adsorber changes. Also, this additional rich cycle requires additional fuel consumption. Accordingly, there exists a need for a NOx adsorber diagnostic system which provides an accurate measure of the stored NOx using measured O2 sensor delay times without requiring an otherwise unnecessary separate lean/rich engine cycle to determine the O2 sensor delay due to the release of stored oxygen. There also exists a need for an engine control system which provides closed loop control of the amount of engine-out NOx supplied to the NOx adsorber between regenerations in accordance with the continuously variable NOx handling capability of the NOx adsorber and which therefore minimizes the tailpipe NOx emissions.
The present invention provides an exhaust control system for diagnosing the state of a NOx adsorber and operating an internal combustion engine in a manner so as to minimize the emission of NOx from the NOx adsorber. As with many prior art systems, the engine is operated using a lean/rich cycle in which NOx stored by the NOx adsorber during the lean portion of the cycle is released and catalytically reduced during the rich regeneration portion of the cycle. Once all of the stored NOx has been released, the engine is switched back to lean for the start of another cycle. The system utilizes a first O2 sensor (which can be a switching O2 sensor or wide-range air/fuel sensor) located upstream of the NOx adsorber and a second O2 sensor located downstream of the NOx adsorber.
In accordance with one aspect of the invention, the determination of the amount of NOx stored by the adsorber is based upon a realization that the delay in switching between the upstream and downstream O2 sensors during the rich-to-lean transition is due to the storage of oxygen by the NOx adsorber at the beginning of the succeeding lean period and that this delay can be used to estimate the amount of time that was required to release the stored oxygen during the lean-to-rich transition. Thus, the delay in switching between the upstream and downstream O2 sensors at the end of the rich regeneration period (the O2 storage time) is used as an indication of the amount of switching delay due to oxygen release during the lean-to-rich transition, and this O2 storage time delay is subtracted from the total delay between switching of the sensors during the lean-to-rich transition (the combined NOx/O2 release time) to thereby obtain an accurate estimate of the amount of sensor switching delay due to the release of the NOx alone (the NOx release time). That is:
NOx release time=the combined release timexe2x88x92O2 release time,
where: the O2 release time=the measured O2 storage time.
Once the NOx release time is determined, the amount of NOx released by the adsorber during the rich regeneration can be determined and this amount can be taken as a good estimate of the amount of NOx that was stored during the previous lean period. In this way, the delay in switching of the sensors due to stored oxygen can be determined and eliminated from the NOx calculation without requiring separate lean/rich operating cycles, as are utilized in the system disclosed in the above-noted U.S. Pat. No. 5,713,199.
Accordingly, an accurate estimate of the amount of NOx stored and released during a particular lean period can be determined using the following steps. First, the engine is run at a lean air/fuel ratio for a period of time and then is switched to a rich air/fuel ratio. The combined NOx/O2 release time is then determined based upon the amount of time between detection by the upstream O2 sensor of a rich combustion condition and detection by the downstream O2 sensor of the rich combustion condition. Next, the engine is switched back to operation at a lean air/fuel ratio and the O2 storage time is determined based upon the amount of time between detection by the upstream O2 sensor of a lean combustion condition and detection by the downstream O2 sensor of the lean combustion condition. Then, the NOx release time is determined using the combined NOx/O2 release time and the O2 storage time, preferably by subtracting the O2 storage time from the combined release time. Preferably, the NOx release time is used along with the exhaust flow rate and the magnitude of the air/fuel ratio during regeneration to determine the amount of NOx released (and therefore the amount stored during the previous lean period). Thereafter, operation of the engine at the lean air/fuel ratio is continued for a length of time that is dependent upon the calculated amount of NOx released.
In accordance with another aspect of the invention, this technique for determining the the amount of NOx stored during the previous lean period (i.e., stored NOx amount) can be used as a part of an exhaust control system in which the stored NOx amount is used along with the exhaust gas temperature and other factors to determine the point at which the next rich regeneration will take place. This can be accomplished by using the stored NOx amount to determine a NOx storage limit that represents the amount of engine-out NOx that the adsorber is expected to be capable of handling at a particular efficiency. During the next lean period, the amount of engine-out NOx is then monitored and another rich regeneration is performed when the engine-out NOx reaches this NOx storage limit.
In accordance with yet another aspect of the invention, there is provided a system for determining the storage efficiency of the NOx adsorber and controlling operation of the engine based upon the determined efficiency. The storage efficiency is determined using the amount of NOx stored during the previous lean period and the amount of engine-out NOx produced during that same period. The amount of stored NOx can be determined in the manner described above. The engine-out NOx can be determined in a known manner using such factors as the air flow rate, engine load, engine speed, EGR setting, spark advance, and the magnitude of the air/fuel ratio during the lean period. The NOx adsorber storage efficiency is then used to determine the NOx storage limit that is utilized to determine the point at which the next rich regeneration should begin.
The NOx storage limit can be selected from values stored in memory and can be adjusted up or down depending upon whether the determined storage efficiency of the NOx adsorber is greater than or less than a desired efficiency. Then, when the amount of NOx stored falls below a selected threshold, a sulfur purge can be performed (thereby increasing the actual storage capacity) and, upon detecting that the storage efficiency has increased, the system will automatically respond by increasing the determined NOx storage limit that is used to determine how long the engine will be run lean each cycle. This provides closed loop control of the NOx emissions, allowing the system to utilize whatever storage capacity exists in the NOx adsorber at any one time.